COMPOUND SEMICONDUCTOR DEVICE USING SiC SUBSTRATE AND ITS MANUFACTURE

ABSTRACT

A compound semiconductor device includes: a conductive SiC substrate; an AlN buffer layer formed on said conductive SiC substrate and containing Cl; a compound semiconductor buffer layer formed on said AlN layer which contains Cl, said compound semiconductor buffer layer not containing Cl; and a device constituent layer or layers formed above said compound semiconductor buffer layer not containing Cl.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of an internationalapplication PCT/JP2006/314743 filed on Jul. 26, 2006, the whole contentsof which are incorporated herein by reference.

FIELD

The present invention relates to a compound semiconductor device and itsmanufacture method, and more particularly to a compound semiconductordevice using a SiC substrate and its manufacture method. In addition, asystem using a compound semiconductor device is also related.

The term “gallium nitride (GaN) based compound semiconductor” meansAl_(x)In_(y)Ga_(1-x-y)N (0≦x≦1.0, 0≦y≦1.0).

BACKGROUND

Compound semiconductor devices using GaN or GaN based compoundsemiconductor are under active development. GaN has a wide band gap of3.4 V allowing a high voltage operation. Various types of semiconductordevices can be manufactured by forming a hetero junction of GaN basedcompound semiconductor. Metal organic chemical vapor deposition (MOCVD)is mainly used as a crystal growth method for the GaN based compoundsemiconductor. A method called hydride vapor phase epitaxy (H-VPE) hasbeen under research recently, which method grows nitride semiconductorby reacting HCl and group III metal to form metal chloride which isfurther reacted with ammonia or the like to form nitride semiconductor.

A semiconductor light emitting device using GaN based compoundsemiconductor can emit blue or ultraviolet light, and can form a whitelight source by using phosphors. Various light emitting devices aremanufactured by growing GaN based compound semiconductor crystal on asapphire substrate or on a SiC substrate.

GaN has a high breakdown voltage, and is expected for applications in afield requiring a high voltage operation and a high speed operation suchas high electron mobility transistors (HEMT) used in a mobile phone basestation. In addition, GaN is also expected for inverter/converterrelated systems such as power supply, car control unit, power plant.Various types of GaN-HEMT have been reported having a GaN layer as anelectron transfer or channel layer among GaN/AlGaN crystal layers grownon a substrate such as sapphire, SiC, GaN and Si. A breakdown voltageover 300 V in a current-off state is presently reported. The best outputcharacteristics are now obtained in GaN-HEMT using a SiC substrate. Ahigh thermal conductivity of SiC is considered to contribute to thisperformance. In manufacturing a high speed operation GaN device, asemi-insulating SiC substrate is used to suppress a parasiticcapacitance. However, a price of a semi-insulating single crystal SiCsubstrate is high, which may possibly hinder the spread of GaN basedsemiconductor devices. A conductive SiC substrate is available at alower price than a semi-insulating SiC substrate. However, when asemiconductor device is formed on a conductive SiC substrate, parasiticcapacitance increases.

JP-A-2002-359255 proposes to grow, as an underlying layer, an AlN layerto a thickness of 2 μm on a conductive SiC substrate having resistivitysmaller than 1×10⁵ Ωcm by metal organic chemical vapor deposition(MOCVD) and form device constituent layers on the underlying layer.

JP-A-2003-309071 proposes to grow an AlGaN underlying layer above acrystal substrate such as sapphire and SiC via an AlN low temperaturegrowth buffer layer by MOCVD and form an AlGaN layer on the underlyinglayer.

International Publication No. 2004-524690 proposes a hybrid growthsystem capable of growth in an MOCVD mode and growth in an H-VPE mode inthe same chamber and capable of growth in both the modes.

JP-A-2005-252248 proposes to suppress a growth chamber temperature nothigher than 750° C. not forming deposition and maintain a substrate at atemperature of 900° C. to 1700° C. by high frequency heating, duringH-VPE growth of AlN crystal on a sapphire substrate or on a Sisubstrate.

MOCVD is difficult to make the growth speed high, and is not suitablefor growth of a thick GaN based semiconductor layer. Although it isknown that H-VPE has the merit of a high growth speed, H-VPE is acrystal growth method under development, and there are some futureissues of findings of problematic points, their solutions and the like.

SUMMARY

According to aspects of the present invention, there is provided acompound semiconductor device including:

a conductive SiC substrate;

an AlN layer containing Cl and formed on the conductive SiC substrate;

an AlN layer not containing Cl and formed on the AlN layer containingCl; and

a device constituent layer or layers formed above the AlN layer notcontaining Cl.

According to another aspect of the present invention, there is provideda method for manufacturing a compound semiconductor device comprisingsteps of:

(1) growing a first AlN buffer layer by H-VPE on a conductive SiCsubstrate;

(2) growing a second AlN buffer layer by MOCVD on the first AlN bufferlayer; and

(3) growing a device constituent layer or layers by MOCVD above thesecond AlN buffer layer.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are schematic cross sectional views of GaN basedHEMT's according to a first embodiment and a comparative example, and agraph showing comparison of secular changes in ohmic contactresistances.

FIGS. 2A and 2B are schematic cross sectional views of a hydride VPEsystem and an MOCVD system.

FIGS. 3A and 3B are a sketch of a grown crystal layer and a TEM imagefrom which the sketch was drawn.

FIGS. 4A and 4B show a signal Waveform of EDX used for compositionmeasurements and a table showing compositions measured at four points(two points in AlN and two points in SiC) in a depth direction of asample having an Al layer grown on a SiC substrate by H-VPE.

FIG. 5 is a schematic cross sectional view of a GaN based HEMT accordingto a second embodiment.

FIGS. 6A and 6B show a TEM cross sectional photograph of a laminationstructure according to a modification of the second embodiment, and asignal waveform of EDX measured at a seventh sport.

FIGS. 7A and 7B are microscopic photographs showing a growth surfaceaccording to the modification of the second embodiment.

FIG. 8 is a microscopic photograph showing a growth surface according tothe modification of the second embodiment.

FIG. 9 is a schematic cross sectional view of a GaN based light emittingdiode (LED) according to a third embodiment.

DESCRIPTION OF EMBODIMENTS

A price of a semi-insulating SiC substrate is very high, which maypossibly hinder the spread of GaN-HEMT's. A conductive SiC substrate isavailable at a lower price than a semi-insulating SiC substrate. If GaNbased semiconductor devices having good electrical characteristics canbe formed by using a conductive SiC substrate, this contributes to thespread of GaN based semiconductor devices.

If a conductive SiC substrate is used, it is possible to forminexpensive GaN based compound semiconductor devices having a SiC highthermal conductivity. However, as the conductive substrate is used,parasitic capacitance of the semiconductor device increases, hinderinghigh speed operation. Parasitic capacitance can be reduced byintervening a thick semi-insulting or high resistance compoundsemiconductor layer between the conductive substrate and constituentelements of the semiconductor device.

Metal organic chemical vapor deposition (MOCVD) and hydride vapor phaseepitaxy (H-VPE) are known as representative crystal growth methods forgrowing GaN based compound semiconductor.

FIGS. 2A and 2B schematically show the structures of an H-VPE system andan MOCVD system.

FIG. 2A is a schematic cross sectional view showing the structure of anH-VPE system. An induction heating high frequency coil 31 is woundaround the circumference of a quartz reaction tube 30, and a carbonsusceptor 32 for placing thereon a substrate 1 is disposed inside thereaction tube. Two gas introducing pipes 34 and 35 are coupled to thereaction tube 30 at its upstream end shown at the left in FIG. 2A. A gasexhaust pipe 36 is connected at the downstream end of the reaction tube30. A boat 38 is disposed at the upstream side of the susceptor 32 inthe reaction tube 30, and accommodates therein a source 39 of a groupIII element of compound to be grown. For example, the source 39 is Alfor growing AlN and Ga for growing GaN. Ammonium NH₃ as N source gas isintroduced from the gas introducing pipe 34, and HCl is introduced fromthe gas introducing pipe 35. HCl reacts with the group III source 39 inthe boat 38 to form group III element chloride AlCl. The source gasesAlCl and NH₃ are transported to the substrate 1 and react on thesubstrate surface to grow AlN. Remaining gas is exhausted via the gasexhaust pipe 36 to a harmful gas removal tower.

FIG. 2B is a schematic cross sectional view showing the structure of anMOCVD system. A high frequency coil 41 is disposed outside a quartzreaction tube 40. A carbon susceptor 42 for placing a substrate 1thereon is disposed inside the reaction tube 40. Two gas introducingpipes 44 and 45 are connected to the reaction tube 40 at its upstreamside to supply compound semiconductor source gases. For example, NH₃ asN source gas is introduced from the gas introducing pipe 44, and organicgroup III compound source material such as trimethylaluminum,trimethylgallium or trimethylindium as group III element source gas isintroduced from the introducing pipe 45. Crystal growth occurs on asubstrate 1, and remaining gas is exhausted from a gas exhaust tube 46to a harmful gas removal tower. If MOCVD is performed in a low pressureatmosphere, the gas exhaust tube 46 is connected to a vacuum pump and anexhaust port of the vacuum pump is connected to the harmful gas removaltower.

MOCVD has been used widely as a compound semiconductor crystal growthmethod, and can provide good crystallinity. Various techniques forimpurity doping and thickness control have been established. However,growth speed is 1 μm/h at the highest.

H-VPE utilizes chloride as group III element source. Growth speed is asvery high as several ten μm/h. Grown crystal layer has a highpossibility of containing chlorine (Cl) derived from the source gas. Inorder to grow a thick compound semiconductor layer not thinner than 10μm, growth speed of MOCVD is too low, and H-VPE capable of providing ahigh growth speed is suitable.

In forming a GaN-HEMT on a conductive SiC substrate, first, asemi-insulating or high resistance AlN layer having a thickness notthinner than 10 μm, e.g., 20 μm to 50 μm is formed on a conductive SiCsubstrate, preferably by H-VPE. If a thick AlN layer is grown by H-VPE,the advantages of reducing dislocations and improving crystal qualitycan also be obtained.

FIG. 1A is a schematic cross sectional view showing the structure of aGaN-HEMT device of the first embodiment. A first AlN buffer layer 2 isgrown to a thickness of, e.g., about 20 μm by H-VPE on a single crystalconductive SiC substrate 1 having the (0001) plane. The H-VPE systemused is the system shown in FIG. 2A, and the group III element sourcematerial 39 in the boat 38 is Al. The conditions of H-VPE are, forexample, as follows:

Pressure: Atmospheric pressure

Gas flow rate: HCl: 100 ccm (cubic centimeter per minute), NH₃: 10 LM(litter per minute)

Temperature: 1100° C.

Resistivity of the first AlN buffer layer 2 can be set considerablyhigher than, for example, 1×10⁵ (represented as 1E5) Ωcm. However, thefirst AlN buffer layer 2 possibly contains chlorine. There is a highpossibility of adverse effects if chlorine reaches, e.g., an electrode.A second AlN buffer layer 3 is grown on the thick first AlN buffer layer2 by MOCVD, and GaN based HEMT constituent layers are grown on thesecond AlN buffer layer 3 by MOCVD.

The MOCVD conditions using the MOCVD system shown in FIG. 2B are set,for example, as follows:

Source material and its flow rate:

Trimethylgallium (TMG): 0 to 50 sccm (standard cubic centimeter perminute)

Trimetylaluminum (TMA)): 0 to 50 sccm

ammonium (NH₃): 20 SLM (standard liter per minute)

n-type impurities: silane (SiH₄)

p-type impurities: bis-cyclopentadienyl magnesium (Cp2Mg)

Pressure: 100 torr

Temperature: 1100° C.

TMA and NH₃ are supplied to grow a second AlN buffer layer 3 notcontaining chlorine and having a thickness of, e.g., 300 nm by MOCVD onthe first AlN layer 2 grown by H-VPE and possibly containing chlorine.Next, TMG and NH₃ are supplied (TMA not supplied) to grow a GaN layer 4having a thickness of, e.g., 3 μm by MOCVD on the second AlN bufferlayer 3. This GaN layer 4 is a non-doped layer and constitutes a regionof an active layer (electron transfer layer) for transportingtwo-dimensional electron gas.

Continuously with the growth of the GaN layer 4, TMA as Al source gas isadded by 5 sccm to grow a non-doped AlGaN layer 5 having a thickness of,e.g., 5 nm, and then by introducing also silane SiH₄ as source gas forn-type impurity Si, an n-type GaN layer 6 doped with Si at 4E18 cm⁻³ isgrown to a thickness of 20 nm. The non-doped AlGaN layer 5 is used as aspacer layer for separating the n-type AlGaN layer 6 from the activelayer 4. The n-type AlGaN layer 6 is used as an electron supply layerfor supplying electrons as carriers to the active layer 4. In thismanner, the basic structure of HEMT is formed. Essentially, each crystallayer grown by MOCVD does not contain Cl.

By stopping the supply of TMA, an n-type GaN layer 7 having a thicknessof 7 nm and doped with Si at about, e.g., 5E18 cm⁻³ is grown as aprotective layer on the n-type AlGaN layer 6. The n-type AlGaN layer 6is covered with the n-type GaN layer 7 having a smooth surface.

The substrate is taken out of the MOCVD system, and an isolation regionis formed by recess etching using BCl₃. A recess having a depth of,e.g., about 100 nm is formed for element isolation, extending from thesurface through the n-type GaN layer 7, n-type AlGaN layer 6 andnon-doped AlGaN layer 5, and partially entering the non-doped GaN layer4. After element isolation, an SiN film 8 is deposited on the substratesurface by plasma CVD. Openings are formed through the SiN film 8 insource/drain electrode contact regions, and the n-type GaN layer 7 isetched and removed by dry etching using Cl₂. A source electrode S and adrain electrode D are formed by depositing a Ta layer having a thicknessof, e.g., 10 nm and depositing an Al layer having a thickness of, e.g.,300 nm on the Ta layer, and performing, for example, lift-off. Annealingis performed at 600° C. to form ohmic contacts. The SiN film 8 in thegate contact area is etched, and a gate electrode is formed bydepositing a Ni layer having a thickness of, e.g., 20 nm and an Au layerhaving a thickness of, e.g., 400 nm and performing lift-off. The gateelectrode forms a Schottky contact.

By growing the semi-insulating AlN layer 2 thick, at least to athickness of 10 μm, it can be expected that parasitic capacitance ofHEMT can be suppressed. The AlN layer has the advantages that as itgrows, dislocations reduce and crystallinity improves. From thisviewpoint, it is particularly preferable to grow the AlN layer having athickness of 20 μm or more. Although a thickness upper limit isdetermined by a warp and crack of a wafer, an upper limit may be set to,for example, 50 μm. A current collapse phenomenon that an on-resistancechanges during operation can be avoided by forming the GaN protectivelayer 7 and SiN layer 8 on and above the n-type AlGaN electron supplylayer 6. Since SiC has a high thermal conductivity, it is expected thata high breakdown voltage and high power operation can be realized.

FIG. 3A is a sketch of an image of the first AlN buffer layer 2, secondAlN buffer layer 3 and GaN layer 4 grown on and above the SiC substrate1 observed with TEM. FIG. 3B is a TEM image from which the sketch wasdrawn. Since the first AlN buffer layer 2 is thick, the intermediateregion is omitted. As the first AlN buffer layer 2 is grown,dislocations stretch in a vertical direction and its density reducesrapidly as the thickness increases. It was observed that as an AlN layerwas grown to a thickness of about 20 μm by H-VPE, dislocation densityreduced from about 1E10 cm⁻² to about 1E5 cm⁻². As the AlN layer 3 isgrown by MOCVD on the AlN layer 2 grown by H-VPE, it is observed thatdislocations are formed in a lateral direction. Namely, the AlN layer 2by H-PVE has the physical properties quite different from those of theAlN layer 3 by MOCVD.

It was checked whether chlorine was captured when the AlN layer 2 isgrown on the SiC substrate 1 by H-VPE. An energy dispersive X-rayfluorescence spectrometer (EDX) was used for measurements.

FIG. 4A shows a signal waveform of EDX. A Si peak is observed to theright of an Al peak, and a Cl peak is observed to the right of the Sipeak. Compositions were measured by changing the depth, two points inthe AlN layer and two points in the SiC substrate.

FIG. 4B is a table showing a list of compositions at four measurementpoints. C and N were excluded from measurement objects, and relativecomposition is calculated using a total sum of 100% of Si, Al and Cl. Itcan be understood that at any of four points, Si, Al and Cl are observedand Cl is diffused not only in the grown AlN layer 2 but also in the SiCsubstrate. Cl composition of 1.28% in the AlN layer 2 corresponds to aconcentration of about 1E20 cm⁻³, and Cl composition of 0.31%corresponds to a concentration of about 2E19 cm⁻³. It indicates that theAlN layer grown by H-VPE contains Cl at a high concentration. It canalso be understood that Al diffuses in the SiC substrate and Si diffusesin the AlN layer.

FIG. 1B is a cross sectional view showing the structure of GaN basedHEMT according to the comparative example. As compared to the firstembodiment, the second AlN buffer layer 3 is not grown. Similar to thefirst embodiment, a first AlN buffer layer 2 is grown to a thickness ofabout 20 μm on a single crystal conductive SiC substrate 1 having the(0001) plane, by H-VPE. The substrate is transported into a MOCVD systemand a non-doped GaN layer having a thickness of 3 μm is grown on thefirst AlN buffer layer 2 by MOCVD.

Thereafter, similar to the first embodiment, following the growth of theGaN layer 4, supply of TMA as Al source gas starts to grow a non-dopedAlGaN layer 5 having a thickness of, e.g., 5 nm, and then by introducingalso silane (SiH₄) as source gas for n-type impurity Si, an n-type GaNlayer 6 doped with Si at 4E18 cm⁻³ is grown to a thickness of 20 nm. Bystopping the supply of TMA, an n-type GaN layer 7 having a thickness of7 nm and doped with Si, e.g., at about 5E18 cm⁻³ is grown as aprotective layer on the n-type AlGaN layer 6. The substrate is taken outof the MOCVD system, and an isolation region is formed by recess etchingusing BCl₃. After element isolation, an SiN film 8 is deposited on thesubstrate surface by plasma CVD. Openings are formed through the SiNfilm 8 in the source/drain electrode contact regions, and the n-type GaNlayer 7 is etched and removed. A source electrode S and a drainelectrode D are formed by depositing a Ta layer having a thickness of 10nm and depositing an Al layer having a thickness of 300 nm on the Talayer, and performing lift-off. Annealing is performed at 600° C. toform ohmic contacts. The SiN film 8 in the gate contact area is etched,and a gate electrode forming a Schottky contact is formed by depositingan Ni layer having a thickness of, e.g., 20 nm and an Au layer having athickness of, e.g., 400 nm and performing lift-off.

Burn-in test was conducted at 250° C. for GaN based HEMT's of the firstembodiment and comparative example to check a secular change in contactresistances of ohmic electrodes.

FIG. 1C is a graph showing the results of the burn-in test. The abscissarepresents a burn-in time in the unit of hour (h), and the ordinaterepresents contact resistance in the unit of Ωcm. The contact resistanceis preferably maintained in a range of ±15%. Curve c0 indicates thecharacteristics of the comparative example shown in FIG. 1B not havingthe second AlN buffer layer. The contact resistance increases rapidlyand suddenly from 1E-7 to about 1E-4 in a burn-in time of about 2 hours.A lifetime at 250° C. is about 2 hours. It is considered that thislifetime is too short for practical use. It is considered that the ohmicelectrode was damaged because of Cl diffusion shown in FIGS. 4A and 4B.By using H-VPE, a thick high resistance AlN buffer layer can be grown onan inexpensive (0001) conductive SiC substrate and the parasiticcapacitance can be reduced. However, as a GaN device is formed on theAlN layer grown in this manner, it has been found that the contactresistance increases rapidly in a short time, resulting in a defectivedevice.

Curve c1 indicates the characteristics of a sample of the firstembodiment shown in FIG. 1A in which the second thin AlN buffer layer 3is grown by MOCVD on the first thick AlN buffer grown by H-VPE. Contactresistance increases rapidly in burn-in time of about 800 hours.Lifetime at 250° C. is about 800 hours extending the lifetime of thecomparative example by about 400 times.

It can be considered from the structures of grown layers shown in FIGS.3A and 3B that as the AlN layer 3 is grown by MOCVD on the AlN layer 2grown by H-VPE, vertical growth of dislocations are blocked by MOCVD,and dislocations are deflected to lateral or horizontal directions inMOCVD-grown layers, and that Cl diffusions are blocked thereby. Cl whichcannot be blocked by the GaN layer having a thickness of about 3 μmcould be blocked significantly by the AlN layer 3 having a thickness of300 nm. It can be considered that the AlN layer grown by MOCVD and notcontaining Cl functions as a Cl diffusion blocking layer. Clconcentration in the AlN layer 3 grown by MOCVD was not higher than 1E15cm⁻³ at least in the upper region of the layer, and this concentrationis negligible as compared to Cl concentration in the AlN layer grown byH-VPE. This state is described as “not containing Cl”. It can beconsidered that the Cl diffusion blocking effect of the AlN layer may beinfluenced greatly by the presence of an interface. In addition, Oxygenat the interface between AlN layer 3 grown by MOCVD and AlN layer 2grown by H-VPE disturbs Cl diffusion. Use of lamination of AlN and ALGaNlayers is considered in place of a single AlN layer. FIG. 5 is aschematic cross sectional view showing the structure of a GaN based HEMTaccording to the second embodiment. In place of the AlN layer 3 of thefirst embodiment having a thickness of 300 nm and grown by MOCVD, an AlNlayer 13 x of 100 nm thick and an AlGaN layer 13 y of 200 nm thick weregrown by MOCVD. Other structures are similar to those of the firstembodiment. A sample of the second embodiment was formed. The effects ofdeflecting dislocations to lateral directions were observed also at aninterface of AlN/AlGaN. Similar to the sample of the first embodiment, aburn-in test was conducted for the sample of the second embodiment tomeasure contact resistance of an ohmic electrode.

Curve c2 in FIG. 1C indicates the characteristics of samples of thesecond embodiment. Burn-in time maintaining good state extended to 1600hours at 250° C. This burn-in time means, as converted to a temperatureof 150° C., burn-in time of about 20 years. It can be considered thatthe hetero interface of AlN/AlGaN blocks Cl diffusion further. The AlGaNlayer grown on the AlN layer may be an AlGaInN layer. It is expectedthat In addition will deflect dislocations further and Cl diffusion willbe blocked further.

In the second embodiment, the AlN layer having a thickness of 100 nm andthe AlGaN layer having a thickness of 200 nm were laminated. It isconsidered that there are various thicknesses of the layers realizingthe Cl diffusion blocking effects.

FIG. 6A shows a TEM photograph of a device which, on an AlN layer grownby H-VPE, laminates an AlN layer having a thickness of 150 nm and anAlGaN layer having a thickness thinner than 20 nm, by MOCVD, and grows aGaN layer on the lamination. An initial growth layer of AlGaN layer isobserved as having a stiff or uneven surface. This may be considered asa change in directions of dislocations. It can be considered that thisinitial growth layer has the large diffusion blocking effects. Clconcentration was measured at points shown in FIG. 6A with EDX, and thedetected concentrations were all under the detection limit.

FIG. 6B shows an EDX spectrum measured at the spot 7 shown in FIG. 6A.Any Cl peak is not observed. It can be expected that Cl diffusion can beblocked satisfactorily even if thinner diffusion blocking layers areused.

The AlN layer grown by H-VPE has a gentle irregular surface visuallyobservable although this surface is difficult to be observed by TEM.

It has been found that irregularity of the surface is reduceddrastically and the surface is planarized, as the AlN layer and furtherAlGaN layer are grown by MOCVD on the AlN layer grown by H-VPE. It canbe considered that as a layer is grown by MOCVD on the AlN layer grownby H-VPE, irregularity is relaxed. With a greater possibility, a layerto be grown by MOCVD is not limited to the AlN layer.

FIG. 7A is an atomic microscope (AFM) top view of the surface of an AlNlayer grown by H-VPE, indicating an area of a 5 μm square. A rectanglein FIG. 7A is an area of a 2.5 μm square. The surface is irregular,having a largest step of about 40 nm.

FIG. 7B is an AFM top view of the surface of a GaN layer grown by MOCVDon the AlN layer grown by H-VPE shown in FIG. 6A, indicating an area ofa 5 μm square. The largest step reduced to about 2 nm. It can be seenthat the surface planarizing effects can be obtained even if the GaNlayer is grown by MOCVD on the AlN layer grown by H-VPE. This isconsidered as a modification of the second embodiment.

The surface planarizing effects by MOCVD growth following H-VPE growthare expected to provide large characteristic improving effects forsemiconductor devices whose characteristics are influenced by surfaceirregularity.

For optical devices, a GaN based compound semiconductor layer having athickness of about 3 to 4μm is often used. As the modification of thesecond embodiment, an AlN layer having a thickness of 20 to 25 μm wasgrown by H-VPE on a conductive SiC substrate, and an AlN layer having athickness of 100 nm, an AlGaN layer having a thickness of 200 nm andfurther a GaN layer having a thickness of 3 to 4 nm were grown by MOCVDon and above the H-VPE grown AlN layer.

FIG. 8 shows an AFM top view of the surface of the GaN layer, indicatingan area of a 20 μm square. A planarized surface without spots isobserved and it can be considered that dislocations are reduced.

FIG. 9 is a schematic cross sectional view showing the structure of anultraviolet light emitting diode (LED) according to the thirdembodiment. An AlN layer 2 is grown by H-VPE to a thickness of 20 μm ona conductive single crystal SiC substrate 1. Then, successive growth isdone by MOCVD. Namely, an AlN layer 13 x is grown to a thickness of 100nm, an n-type AlGaN layer 14 doped with Si at 4E18 cm⁻³ is grown to athickness of 500 nm, a non-doped AlGaN layer 15 is grown to a thicknessof 100 nm, and a p-type AlGaN layer 16 doped with Mg Si at 4E18 cm⁻³ isgrown to a thickness of 100 nm. A band gap of the non-doped AlGaN layer15 is set narrower than those of the doped AlGaN layers 14 and 16 onboth sides thereof to realize the structure of an emission layersandwiched between clad layers. The n-type AlGaN layer 14 serves as then-type region of LED and is considered as functioning as the layer forblocking Cl diffusion and planarizing the surface similar to the AlGaNlayer 13 y of the second embodiment.

After MOCVD, the substrate is taken out of the MOCVD system, recessetching is performed to an intermediate depth of the n-type AlGaN layerto expose the n-type AlGaN layer and achieve element isolation. An SiNlayer 17 is formed on the substrate surface by plasma CVD. Openings areformed by Cl₂ etching using a resist mask, and an anode electrode A ofan Ni/Au lamination and a cathode electrode K of a Ta/Al lamination areformed.

According to this embodiment, by growing the AlN layer 13 x and AlGaNlayer 14 by MOCVD on the AlN layer 2 grown by H-VPE, the surfaceplanarizing effects can be obtained, dislocation density can be reducedto an order of 1E6 cm⁻², which is about two digits lower than aconventional case, and Cl diffusion can be suppressed so that Clconcentration is not higher than 1E15 cm⁻³. It has been found that thereliability performance of ultraviolet LED has made the lifetimeextended from 1,000 hours to 10,000 hours. The AlGaN layer on the AlNlayer by MOCVD may be replaced with an AlGaInN layer. It can beconsidered that addition of In improves the flatness further, reducesdislocations, and improves the reliability further. By selecting thecomposition of AlGaInN, a blue LED can be formed similar to theultraviolet LED. Instead of an i-type active layer of the pin structure,a quantum well structure, a multiple quantum well structure, a quantumdot structure or the like may also be used.

A light emitting device does not always require a high thermalconductivity of a SiC substrate. The substrate may be required to havetransparency at a emission wavelength. It can be considered from thisviewpoint that a sapphire substrate may be used instead of the SiCsubstrate. The sapphire substrate has lattice mismatch with GaN basedcompound semiconductor, and GaN based compound semiconductor crystalgrown on the sapphire substrate has dislocations. It is expected thatdislocations will be reduced considerably by growing a thick AlN layerby H-VPE on the sapphire substrate. The AlN layer by H-VPE contains Cl,and the characteristics could be thought to be adversely affected.However, by growing an AlN layer by MOCVD on the AlN layer grown byH-VPE, it is expected that Cl diffusion can be blocked considerably. Ifan AlN layer and an AlGaN layer (or AlGaInN layer) are laminated byMOCVD on the AlN layer grown by H-VPE, it is expected that the surfacecan be planarized and Cl diffusion can be suppressed further.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. Although manufacturing a GaN based HEMT and LED have beendescribed, electronic devices to be manufactured are not limited to HEMTand LED. Other electronic devices and optical devices may also bemanufactured. Further, although growing a device constituent layer byMOCVD has been described, other method such as molecular beam epitaxy(MBE) may also be used. It will be apparent to those skilled in the artthat other various modifications, improvements, combinations, and thelike can be made.

The above-described embodiments are applicable to a GaN based compoundsemiconductor device and its manufacture method.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions, nor does theorganization of such examples in the specification relate to a showingof the superiority and inferiority of the invention. Although theembodiment(s) of the present invention(s) has(have) been described indetail, it should be understood that the various changes, substitutions,and alterations could be made hereto without departing from the spiritand scope of the invention.

1. A compound semiconductor device comprising: a conductive SiCsubstrate; an AlN buffer layer formed on said conductive SiC substrateand containing Cl; a compound semiconductor buffer layer formed on saidAlN layer which contains Cl, said compound semiconductor buffer layernot containing Cl; and a device constituent layer or layers formed abovesaid compound semiconductor buffer layer not containing Cl.
 2. Thecompound semiconductor device according to claim 1, wherein said AlNbuffer layer containing Cl has a thickness not thinner than 10 μm. 3.The compound semiconductor device according to claim 1, wherein saidcompound semiconductor buffer layer is an AlN layer.
 4. The compoundsemiconductor device according to claim 1, wherein said compoundsemiconductor buffer layer is a GaN layer.
 5. The compound semiconductordevice according to claim 3, further comprising an AlGaInN layer orAlGaN layer not containing Cl and formed on said compound semiconductorbuffer layer.
 6. The compound semiconductor device according to claim 3,wherein said device constituent layers include a non-doped GaN layer, anon-doped AlGaN layer and an n-type AlGaN layer, and constitute a HEMT.7. The compound semiconductor device according to claim 6, wherein saiddevice constituent layers further include an n-type GaN layer formed onsaid n-type AlGaN layer and an SiN layer formed on said n-type GaNlayer.
 8. A method for manufacturing a compound semiconductor devicecomprising steps of: (1) growing a first AlN buffer layer by H-VPE on aconductive SiC substrate; (2) growing a second buffer layer of compoundsemiconductor by MOCVD on said first AlN buffer layer; and (3) growing adevice constituent layer or layers above said second buffer layer. 9.The method for manufacturing a compound semiconductor device accordingto claim 8, wherein said second buffer layer is an AlN layer.
 10. Themethod for manufacturing a compound semiconductor device according toclaim 8, wherein said second buffer layer is a GaN layer.
 11. The methodfor manufacturing a compound semiconductor device according to claim 8,wherein said step (1) grows said first AlN buffer layer to a thicknessnot thinner than 10 μm.
 12. The method for manufacturing a compoundsemiconductor device according to claim 9, further comprising betweensaid steps (2) and (3) a step of: (4) growing an AlGaN layer by MOCVD onsaid second AlN buffer layer.
 13. The method for manufacturing acompound semiconductor device according to claim 9, wherein said step(3) grows a non-doped GaN layer, a non-doped AlGaN layer and an n-typeAlGaN layer to form a HEMT structure.
 14. An optical compoundsemiconductor device comprising: a substrate of conductive SiC orsapphire; an AlN buffer layer formed on said substrate and containingCl; a compound semiconductor buffer layer formed on said AlN layer whichcontains Cl, said compound semiconductor buffer layer not containing Cl;and an optical device constituent layer or layers formed above saidcompound semiconductor buffer layer not containing Cl.
 15. The opticalcompound semiconductor device according to claim 14, wherein a surfaceof said compound semiconductor butter layer is more flat than a surfaceof said AlN buffer layer.
 16. The optical compound semiconductor deviceaccording to claim 14, wherein said compound semiconductor buffer layeris an AlN layer.
 17. The optical compound semiconductor device accordingto claim 14, wherein said compound semiconductor buffer layer is a GaNlayer.
 18. The optical compound semiconductor device according to claim16, wherein said optical device constituent layers include a firstAlGaInN layer not containing Cl and formed on said compoundsemiconductor buffer layer.
 19. The optical compound semiconductordevice according to claim 18, wherein said first AlGaInN layer is dopedwith impurities of a first conductivity type, and said optical deviceconstituent layers further include a non-doped AlGaInN emission layerformed on said AlGaInN layer and a second AlGaInN layer formed on saidnon-doped emission layer and doped with impurities of a secondconductivity type opposite to the first conductivity type.
 20. Theoptical compound semiconductor device according to claim 19, whereinsaid AlGaInN emission layer has a band gap narrower than those of saidfirst and second AlGaInN layers.